1. Field of the Invention
The present invention relates to a thermal flow sensor for detecting a fluid such as air by utilizing a thermal resistor and heating resistor, etc.
2. Description of the Prior Art
A flow sensor of such a type which detects a flow rate from a thermally balanced condition of a bridge circuit including a heating resistor and which is provided in a fluid such as air is known. The structure of a conventional air flow sensor using a platinum wire as a heating resistor will be explained with reference to FIG. 1 and FIG. 2. FIGS. 1(a) and 1(b) are vertical sectional side elevation and front elevation views of a conventional thermal flow sensor. FIG. 2 is a circuit diagram indicating a temperature control circuit of the conventional thermal flow sensor.
In FIGS. 1(a) and 1(b), the conventional thermal flow sensor is provided with a measurement tube (2) supported by a supporting member (3) at a predetermined position within a housing (1) forming the main path for a fluid such as air. A plurality of heating wire supporting members (4) are provided at the inner surface of measurement tube (2). A platinum wire (5) is supported in a plane orthogonal to the flow of air by the heating wire supporting members (4). An air temperature sensor (6) is provided within the measurement tube (2). Electrical connecting lead wires of the platinum wire (5) and the air temperature sensor (6) are extended to the inside of a control circuit (7) provided on the outer surface of the housing (1) by way of through-holes (not illustrated) formed respectively in the measurement tube (2), the supporting member (3) and the housing (1), and are connected with a temperature control circuit provided within the control circuit (7). Protection nets (8) and (9) are provided at both ends of the housing (1).
In FIG. 2, the temperature control circuit (10) has a bridge circuit formed by the platinum wire (5), the air temperature sensor (6) and resistors (11) and (12) and both input terminals of a differential amplifier (13) are connected to the intermediate connecting points b and f of the bridge circuit. An output terminal of the differential amplifier (13) is connected to the base of a transistor (14), while the emitter of the transistor (14) is connected to the connecting point a of the bridge circuit with the collector of the transistor (14) being connected to a positive terminal of a DC power supply (15). The temperature control circuit (10) executes temperature control so that the bridge circuit is capable of maintaining a predetermined thermally balanced state. It is assumed that a resistance value of the platinum wire (5) is defined as Rh, a resistance value of the air temperature sensor (6) as Rc and resistance values of the resistors (11) and (12) as R.sub.1, R.sub.2.
Next, operations of the conventional thermal flow sensor will be explained with reference to FIG. 3, which shows characteristics indicating operations of the conventional thermal flow sensor. In FIG. 3, the horizontal axis indicates an air flow rate, while the vertical axis indicates an error (%).
Operation of the temperature control circuit (10) is well known and, as such, explanation thereof is omitted here. When the voltages at the connecting points b and f become equal, the bridge circuit attains a balanced state. At this time, a current I.sub.h corresponding to the flow rate flows through the platinum wire (5) and the voltage V.sub.h of the connecting point b becomes equal to I.sub.h .times.R.sub.2. This voltage V.sub.h is used as a flow rate signal.
In order to correct errors in a detected flow rate due to scattering in resistance values and resistance-temperature coefficients of the platinum wire (5) and the air temperature sensor (6) and resistance values of the resistors (11) and (12), it is usual for a detected flow rate characteristic to be transformed in parallel by adjusting the resistance value R.sub.1 of the resistor (11) to set a detected value of a predetermined flow rate (usually, a comparatively low flow rate value) to a target value.
FIG. 3 shows a detected flow rate characteristic explaining such compensation, where the resistance value R.sub.1 of the resistor (11) is adjusted so that a characteristic curve .beta. before adjustment by way of the resistor (11) is set within the range of the target value X at a predetermined flow rate Q.sub.1.
In such a thermal flow sensor including the temperature control circuit (10), the resistance value R.sub.1 of the resistor (11) is adjusted to improve accuracy in measurement. However, it would be impossible to adjust unevenness in size of the housing (1) and the measurement tube (2), scattering of these elements in a relative position, deviation of a center axis of the measurement tube (2) from the direction of flow of the fluid, and a gradient of a flow rate characteristic mainly resulting from scattering and deviation in structure and size of the platinum wire (5) (dependence of deviation from a center value of detected characteristic in each flow rate on a flow rate). Further, measurement accuracy cannot be improved in a flow rate at a point other than the adjusted flow rate point Q.sub.1 explained above, particularly, in a flow rate at a point separated far from the adjusted flow rate point Q.sub.1. Accordingly, a gradient of a flow rate characteristic is adjusted, in addition to the adjustment by the resistor (11) explained above.
An adjustment of the gradient of a flow rate characteristic executed in the prior art will be explained with reference to FIG. 4 and FIGS. 5(a)-5(c). FIG. 4 is a circuit diagram indicating a gradient correcting circuit of a conventional thermal flow sensor, while FIGS. 5(a)-5(c) and and FIGS. 6(a)-6(c) are characteristic diagrams indicating operation of the gradient correcting circuit of the prior art.
In FIG. 4, the gradient correcting circuit comprises a substracting circuit (16), a voltage dividing circuit (23), an amplifying circuit (26) and an arithmetic circuit (31).
The substracting circuit (16) comprises resistors (17), (18), (20), (21) and (22) and an operational amplifier (19). A non-inverting input terminal of the operational amplifier (19) is grounded through the resistor (18) and is also connected through the resistor (17) to the connecting point b of the bridge circuit of the temperature control circuit (10). A preset voltage V.sub.ref is applied through the resistor (20) to an inverting input terminal of the operational amplifier (19). The resistor (21) is connected between the inverting input terminal and an output terminal of this operational amplifier (19) and this output terminal is grounded through the resistor (22). The resistance values of the resistors (17), (18), (20), (21) and (22) are defined respectively as R.sub.3, R.sub.4, R.sub.5, R.sub.6 and R.sub.7.
The voltage dividing circuit (23) divides an output voltage V.sub.1 of the substracting circuit (16) with resistors (24) and (25). A series circuit of these resistors (24) and (25) is connected between an output terminal of the substracting circuit (16) and the ground. The resistance values of the resistors (24) and (25) are defined respectively as R.sub.8 and R.sub.9.
An output voltage V.sub.2 of the voltage dividing circuit (23) obtained at the connecting point between the resistors (24) and (25) is applied to the non-inverting input terminal of the operational amplifier (27) in the amplifying circuit (26). The inverting input terminal of the operational amplifier (27) is grounded through a resistor (28). An output voltage V.sub.3 of the amplifying circuit (26) is input to the non-inverting input terminal of an operational amplifier (34) through a resistor (33) within the arithmetic circuit (31). The resistance values of the resistors (28), (29) and (30) are defined respectively as R.sub.10, R.sub.11 and R.sub.12.
The arithmetic circuit (31) comprises resistors (32), (33), (35) and (36) and the operational amplifier (34). The output voltage V.sub.1 of the subtracting circuit (16) is input to an inverting input terminal of the operational amplifier (34) through the resistor (35) and the resistor (36) is connected between an output terminal and the inverting input terminal of the operational amplifier (34). Moreover, a non-inverting input terminal of the operational amplifier (34) is connected through the resistor (32) to the connecting point b of the bridge circuit of the temperature control circuit (10). A voltage V.sub.0 is output from the output terminal of the operational amplifier (34). The resistance values of the resistors (32), (33), (35) and (36) are defined respectively as R.sub.13, R.sub.14, R.sub.15 and R.sub.16.
Operation of each circuit described above will be explained with reference to FIGS. 5(a)-5(c).
The output voltage V.sub.1 of the subtracting circuit (16) satisfies the following equation in accordance with the resistance values R.sub.3, R.sub.4, R.sub.5 and R.sub.6 of the resistors (17), (18), (20) and (21): EQU V.sub.1 ={R.sub.4 /(R.sub.3 +R.sub.4)}.times.{(R.sub.5 +R.sub.6)/R.sub.5 }.times.V.sub.h -{(R.sub.6 /R.sub.5).times.V.sub.ref }
When the resistance values are set adequately, for example, such as R.sub.3 =R.sub.4 and R.sub.5 =R.sub.6, the following result can be obtained: EQU V.sub.1 =V.sub.h -V.sub.ref
Since the operational amplifier (19) is operated only with the power supply voltage having positive polarity, the output voltage V.sub.1 of the operational amplifier (19) does not become negative, and, when V.sub.h &lt;V.sub.ref, V.apprxeq.0, showing a characteristic V.sub.1 indicated in FIG. 5(a). (V.sub.1 becomes about 0.3 V due to an output voltage saturation characteristic of the operational amplifier (19), but, in this case, V.sub.1 is defined here to be nearly equal to zero.)
The output voltage V.sub.2 of the voltage dividing circuit (23) is expressed by the following equation in accordance with the resistance values R.sub.8 and R.sub.9 of the resistors (24) and (25) in the voltage dividing circuit (23): EQU V.sub.2 ={R.sub.9 /(R.sub.8 +R.sub.9)}.times.V.sub.1 (V.sub.1 .apprxeq.0, when V.sub.h &lt;V.sub.ref).
The voltage V.sub.2 is input to the amplifying circuit (26) and the output voltage V.sub.3 of the amplifying circuit (26) is expressed as follows in accordance with the resistance values R.sub.10 and R.sub.11 of the resistors (28) and (29): ##EQU1##
The arithmetic circuit (31) receives the output voltage V.sub.h of the temperature control circuit (10), the output voltage V.sub.3 of the amplifying circuit (26) and the output voltage V.sub.1 of the subtracting circuit (16) and the output voltage V.sub.0 satisfies the following equation in accordance with the resistance values R.sub.13, R.sub.14, R.sub.15 and R.sub.16 of the resistors (32), (33), (35) and (36): ##EQU2##
When the resistance values are set to adequate values, for instance, such as R.sub.13 =R.sub.14 and R.sub.15 =R.sub.16, then the following result can be obtained: EQU V.sub.0 =V.sub.h +V.sub.3 -V.sub.1
From the equations of those output voltages V.sub.1, V.sub.2, V.sub.3 and V.sub.h and the preset voltage V.sub.ref, the following equation can be obtained: EQU V.sub.0 =V.sub.h +{{R.sub.9 /(R.sub.8 +R.sub.9)}.times.{(R.sub.10 +R.sub.11)/R.sub.10 }-1}.times.(V.sub.h -V.sub.ref)(V.sub.0 .apprxeq.V.sub.h when V.sub.h &lt;V.sub.ref)
In the above equation, when the resistance values R.sub.8, R.sub.9, R.sub.10 and R.sub.11 are set to adequate values, for instance, R.sub.8 =R.sub.9 and R.sub.10 =R.sub.11 .times.(1.+-..alpha.), then ##EQU3##
Therefore, when V.sub.h &lt;V.sub.ref, the output voltage of the arithmetic circuit (31) V.sub.0 =V.sub.h irrespective of the resistance values R.sub.10 and R.sub.11, and, when V.sub.h &gt;V.sub.ref, a value obtained by multiplying the value (V.sub.h -V.sub.ref) with a factor determined by the ratio of the resistance values R.sub.10 and R.sub.11 is added to or subtracted from V.sub.h, particularly, when R.sub.10 =R.sub.11, V.sub.0 =V.sub.h irrespective of which V.sub.h and V.sub.ref is larger.
FIG. 5(a) is a diagram indicating characteristics of the voltages V.sub.0, V.sub.1, V.sub.2 and V.sub.h described above. The output voltage V.sub.3 of the amplifying circuit (26) changes in response to the output voltage V.sub.1 of the subtracting circuit (16) in accordance with a value determined by the ratio of the resistance values R.sub.10 and R.sub.11. The output voltage V.sub.0 of the arithmetic circuit (31) is nearly equal to V.sub.h when V.sub.h &lt;V.sub.ref, and changes on the basis of the characteristics of V.sub.0 =V.sub.h when V.sub.h &gt;V.sub.ref.
FIG. 5(b) is a diagram indicating a relationship between an air flow rate and the output voltage V.sub.h of the temperature control circuit (10) and the output voltage V.sub.0 of the arithmetic circuit (31). FIG. 5(c) is a diagram indicating a relationship between an air flow rate and a detected error in air flow rate which depends on the output voltages V.sub.h and V.sub.0. As shown in FIG. 5(b), the output voltage V.sub.0 of the arithmetic circuit (31) can be arbitrarily changed to a plus (+) or minus (-) side with respect to the output voltage V.sub.h of the temperature control circuit (10) in accordance with the resistance values R.sub.11 and R.sub.12 only when a flow rate is higher than the air flow rate Q.sub.ref corresponding to the preset voltage V.sub.ref.
Accordingly, as shown in FIG. 5(c), an error detected by the output voltage V.sub.h of the temperature control circuit (31) can be adjusted to a +.alpha. side when such an error is negative and to a -.alpha. side when such an error is positive, when a flow rate is larger than the air flow rate Q.sub.ref.
In such a manner as described above, after an error-flow rate characteristic is transformed in parallel so as to enter the range of a target value X at a predetermined flow rate Q.sub.1, the gradient of such a characteristic is corrected with respect to the flow rate Q.sub.ref having a relatively small error and located near the flow rate Q.sub.1 as a flow rate at a reference point.
A conventional gradient correcting circuit of a thermal flow sensor as explained above has such problems as are explained hereunder.
The correcting circuit is constituted such that, when the output voltage V.sub.h of the temperature control circuit (10) is lower than the preset voltage V.sub.ref (V.sub.h &lt;V.sub.ref), the output voltage V.sub.o of the arithmetic circuit (31) becomes equal to the output voltage V.sub.h of the temperature control circuit (10). In an actual operation, when the operational amplifier (19) is saturated, it is necessary to supply simultaneously as a sink current from the output terminal of the operational amplifier (19), a current of V.sub.ref /(R.sub.5 +R.sub.6) through the resistors (20) and (21) from the preset voltage V.sub.ref and a current of V.sub.o /(R.sub.15 +R.sub.16) through the resistors (36) and (35) from the output terminal of the arithmetic circuit (31). Because of the structure of an output circuit of the arithmetic circuit (19) and characteristics of semiconductor elements forming the output circuit, the output voltage V.sub.o does not become 0 when the sink current flows, and a saturated voltage V.sub.sat of about 0.3 V usually remains at the output of the arithmetic circuit. In order to reduce V.sub.sat, a resistor having a relatively small resistance value such as the resistor (22) shown in FIG. 4 is connected as a pull-down resistor between the output terminal of the operational amplifier (19) and the ground.
However, even if such a pull-down resistor (22) is used, the output voltage of the operational amplifier (19) cannot be reduced to zero and a residual voltage V.sub.s of several tens of millivolts still remains.
Explanation of operation of the conventional apparatus made with reference to FIG. 5 is made on the assumption that the residual voltage V.sub.s .apprxeq.0. Due to the influence of a residual voltage V.sub.s, an error E.sub.s resulting from such a residual voltage V.sub.s actually appears as shown in FIGS. 6(a)-6(c). Particularly, when a flow rate detected by the thermal flow sensor is at a very low level (for instance, a flow rate detected during an idle condition in an internal combustion engine), a detected error becomes 1% when a voltage is at a level of about 3 mV-5 mV. Accordingly, an error resulting from a residual voltage V.sub.s must be taken into account.